Interpretive Summary:
Sweet potato [Ipomoea batatas (L.) Lam] is the third most important root/tuber crop and the seventh most important food crop in the world. More than 20 different viruses infect the sweet potatoes and cause diseases on it. Sweet potato viral disease caused by mixed infections of several viruses, including Sweet potato virus G (SPVG) and Sweet potato virus 2 (SPV2), is the most devastating viral disease worldwide and can reduce the yields up to 80-90%. In this study, the complete genomic sequences of SPVG and SPV2 were determined for the first time. Analyses of the genetic information show that the two viruses are distinct species in a virus genus named Potyvirus. The study provides the information necessary for the proper grouping and naming of the viruses, and further characterization of them at the molecular level. It will also assist with the development of better diagnostic techniques for these viruses.

Technical Abstract:
Complete nucleotide sequences of Sweet potato virus G (SPVG) and Sweet potato virus 2 (SPV2) were determined to be 10,800 and 10,731 nucleotides, respectively, excluding the 3’-poly(A) tail in this study. Their genomic organization is typical of potyviruses, encoding a polyprotein which is likely cleaved into ten mature proteins by three viral proteinases. Conserved motifs of orthologous proteins of viruses in the genus Potyvirus are found in corresponding positions of both viruses. Pairwise comparisons of the genomic, deduced polyprotein and individual protein sequences of the two viruses with those of 75 other potyviruses show that P1 and CP of both viruses are significantly larger, with the SPVG CP as the largest (355 amino acids) among all known species of the genus Potyvirus. The extended N-terminal region of the P1 protein is conserved in potyviruses and ipomovirus infecting sweet potato. The C-terminal half of the CP is highly conserved among Sweet potato feathery mottle virus (SPFMV), Sweet potato virus C, SPVG, SPV2 and Sweet potato virus-Zimbabwe. Phylogenetic analysis based on the deduced CP amino acid sequences supports that these five viruses are closely related, and cluster together in the SPFMV lineage. The analysis also reveals that Sweet potato virus Y and Ipomoea vein mosaic virus are grouped with SPV2 as one species, and these two viruses should be consolidated as SPV2. The high genetic diversity among isolates and existence of several genetic variants in a single plant also suggest that both viruses consist of divergent population.